The Mountain Plover (Charadrius montanus) is a shorebird of conservation concern that breeds in the Great Plains and the Great Basin. They have an uncommon uniparental care system where males and females tend separate nests. Although males and females have similar incubation and chick-rearing responsibilities, it is thought that the males arrive first to the breeding grounds in early to mid-April, establish loose territories, and compete for females. Long-term monitoring of this species in north-central Montana has allowed for studies comparing the effectiveness of the two sexes at separately raising young, showing evidence for differences in nest and brood survival.
However, there is still much to be learned about Mountain Plover breeding biology. While there has been much speculation about their uniparental care system and how it provides the opportunity for sequential polyandry, no work has been done at the genetic level to examine it in further detail. The consequences of this species' reproductive decisions can play a role at the evolutionary level and considering its current and future conservation status, any information about factors that can influence population persistence will be important to understand. The age of first reproduction is important in both life-history theory and conservation biology because it can have a large impact on individual fitness, which in turn influences population dynamics. Evolutionary theory predicts that organisms should reproduce as early as they are capable of doing so, although there are potential tradeoffs if breeding is costly. We individually color-banded and resighted 850 flightless plover chicks during the breeding seasons of 1995 to 2010 in Phillips County, Montana. Of these, 115 were found in the study area as adults with 38 individuals observed breeding at age one (33%). Seventeen females out of 41 resighted (42%) were found either tending a nest or brood at age one but only 13 males of 36 resighted (36%) reproduced as one-year-olds. We developed a set of closed robust design multi-state mark-resighting models in Program MARK to estimate the probability of breeding at age one or delaying breeding to a later age, and how this is influenced by an individual's sex and environmental conditions. The model-averaged probability of a Mountain Plover breeding at age one is 0.20 (SE = 0.05), which was not different than the probability of an older non-breeding bird deciding to breed in any given year (0.18, SE = 0.04). Both sex and environmental conditions had weak effects on the decision of Mountain Plovers to breed but in general, females were more likely to be found breeding than males and plovers were more likely to breed in drier years than in wetter years.
The coloration of exposed eggs of ground-nesting birds has long been thought to function as camouflage to reduce predation, with eggs that more closely match the area around the nest having greater survival. We tested this hypothesis using digital photographs of 374 Mountain Plover nests and the substrate surrounding each nest to produce covariates to include in nest survival models. These covariates included values representing the difference between the color of the eggs and that of the substrate, the average egg and substrate colors, and variation in both egg and substrate color. Nest survival decreased as the difference between the color of the eggs and substrate increased (accounted for by two measures, the first in the L*a*b* color space, the second in three-dimensional RGB color space: βΔE= -0.021, SE = 0.024, 95% Confidence Interval: CI -0.069, 0.026 and βΔRGB = -0.004, SE = 0.005, 95% CI -0.013, 0.005) and increased as the variability in color in the substrate surrounding the nest increased (βSDsub= 2.624, SE = 2.846, 95% CI -2.954, 8.202), although after model-averaging these effects were not well-supported. Model-averaged estimates of daily nest survival ranged from 0.90 to 0.98 (unconditional SEs from 0.004 to 0.129).
The hormones prolactin (PRL) and testosterone (T) are linked to parental behaviors such as incubation and brood-rearing in birds. We collected blood from incubating male and female Mountain Plovers in Montana and quantified circulating plasma PRL and T. We examined how these two hormones varied by sex, during the incubation period, and across the nesting season. PRL levels were similar for incubating male and female Mountain Plovers, but incubating males had significantly higher T concentrations than females. There was no relationship between day of incubation and the concentration of circulating PRL for either incubating female plovers or male plovers. The relationship between the Julian day and concentration of circulating PRL was also not strong for either sex. T concentrations tended to decrease throughout the incubation period although there was not a strong relationship with day of incubation for either incubating female or male plovers. There was weak evidence that circulating T for females declined across the breeding season but not in males.
We examined maternal investment in Mountain Plovers by measuring the dimensions of plover eggs in Phillips County, Montana to calculate volumes. We modeled possible differences in egg volume in male- and female-incubated nests in relation to the effects of sex of the incubating adult, Julian day of nest initiation, and drought conditions. We measured >1000 eggs from 194 nests tended by 131 females and from 213 nests of 148 males. Male- and female-incubated eggs had similar volumes (mean volume = 13.20 cm3, SE = 0.03 compared to 13.17 cm3, SE = 0.04) but were significantly different across the breeding season. The eggs in female-incubated nests tended to be larger than those in male-incubated nests early in the breeding season but were smaller as the season progressed, while the volume of male-incubated eggs peaked in the middle of the season. Egg volumes were affected by drought conditions with larger eggs produced during the driest periods of this study. Volumes were similar within nests and were not influenced by the age of the incubating adult. Larger eggs tended to produce larger chicks. The eggs of Mountain Plovers are overall similar in size, even between male- and female-incubated nests and under different environmental conditions.
After mating and laying an initial set of three eggs for a male, a female Mountain Plover has the opportunity to mate with other males, potentially providing eggs for them before laying a set of three eggs that she incubates, then tends the resulting chicks. Courtship activity between multiple males and females within a single breeding season has been documented several times but few copulations have been observed. We examined the prevalence of multiple paternity within male- and female-tended broods using DNA extracted from chicks and the tending adult. During six breeding seasons (2006-2011) we collected blood from 99 incubating adult plovers and from 291 chicks of 115 broods (54 adult males with 154 chicks from 59 broods and 45 adult females with 137 chicks from 56 broods). We found thirteen cases of mixed paternity within male-tended broods and one case of mixed paternity within a female-tended brood. There was weak evidence that male-tended broods with mixed paternity occurred more frequently early in the breeding season.
Our study provides needed information about the reproductive biology and population ecology of a species that has been in decline for more than four decades. Our results support the egg crypsis hypothesis because nests that most closely match their surroundings have greater survival. This study provides further information about the relationship between parental duties and circulating PRL and T and the breeding biology of a shorebird of conservation concern. During the course of this study the area experienced a wide range of environmental conditions, and our results show that Mountain Plovers are well-adapted to breeding in these changeable conditions and that this uncommon parental care system is more stable than originally thought.